How often should you change a shower filter cartridge?

How often to change a shower filter comes down to one rule: follow the maker's schedule, because the media inside a cartridge are used up as water runs through them. Waterhush suggests a new cartridge every 3 months, or every 2 months for households of two or more. That is a suggestion, not a promise of how long a cartridge lasts.

Below, we explain why cartridges run out, what makes one run out sooner, why a date beats a hunch and how to change one. For the wider picture, see our complete, honest guide to filter shower heads in the UK.

We sell a filter shower head and its refills, so every figure here links to its source.

How often to change a shower filter: the short answer

A shower filter cartridge should be changed on its maker's schedule, because its filter media are used up as they work. For our cartridge, we suggest every 3 months, or every 2 months for households of two or more.

The Drinking Water Inspectorate (DWI) gives the same advice for home water filters. Its guidance on domestic water filters and softeners says to change the cartridge in line with the manufacturer's instructions, or it will cease to be effective.

Every schedule is an estimate. A cartridge is used up by the water that runs through it, not by the calendar, so a schedule in months has to assume a typical amount of use. The practical answer is a date: note when you fit a cartridge and change it on schedule, not on a hunch.

Why do shower filter cartridges need replacing?

Shower filter cartridges need replacing because the media inside react with free chlorine, and a cartridge holds only so much media. Once the media have done their work, the cartridge can no longer do its job.

Vitamin C shows the principle clearly. The USDA Forest Service published a technical note on using vitamin C to neutralise chlorine in April 2005. It says two forms of vitamin C, ascorbic acid and sodium ascorbate, neutralise chlorine, and that the reaction is very fast. About 2.5 parts of ascorbic acid are needed to neutralise 1 part of chlorine, and the vitamin C becomes another compound, dehydroascorbic acid.

In other words, vitamin C is consumed as it works. The note was written for operators of water and wastewater systems, not about shower filters, but the chemistry is the same.

KDF is a granular mix of copper and zinc used in filter media. Its maker, Kymera International, says KDF works by redox, a transfer of electrons, and can convert chlorine into chloride. It also says the media can work at high temperatures and suit uses such as filter shower heads.

Our cartridge also contains calcium sulfite and carbon. We explain all four media in what's inside a shower filter: KDF, vitamin C and more.

What capacity means in a filter test

Capacity is the amount of water a filter can treat before it stops meeting a test's pass mark. NSF/ANSI 177, a US standard for shower filtration systems, was described by the trade magazine Plumbing & Mechanical on 1 June 2005. Its free chlorine test runs water carrying 2.0 mg/l of free chlorine through the filter at 40 °C.

A sample passes if the filter cuts that chlorine by 50%, and capacity is set by the last of at least five consecutive passing samples. So capacity is counted in water treated, not in months: a schedule in months turns it into a calendar by assuming how much water you use.

NSF/ANSI 177 is a US standard, and the 2005 description may not match its current edition. We cite it to explain capacity, not as a claim about our cartridge.

What makes a shower filter cartridge run out sooner?

A shower filter cartridge runs out sooner when more water passes through it, or when that water carries more free chlorine. Household size, shower habits and your local supply all play a part.

• More people. A bigger household usually means more showers, which is why our suggestion shortens from every 3 months to every 2 for households of two or more.
• Longer or more frequent showers. More time under the shower means more water through the cartridge.
• More free chlorine in the water. Water carrying more free chlorine gives the media more to react with.

The chlorine level in your supply is not fixed. Bristol Water's page on chlorine says it typically adds 0.5 to 1.0 mg/l at its treatment works, and that by the time water reaches homes the level is typically 0.1 to 0.8 mg/l.

Bristol Water says the level varies with distance from the treatment works, the time of year and the time of day, among other things. Cold water holds chlorine for longer, so winter levels run slightly higher. At the busiest times of day, water moves through the network faster, leaving less time for chlorine to dissipate.

More widely, the DWI's guide to chlorine in tap water says water companies typically keep the residual disinfectant at 0.5 mg/l or less. So two homes on the same schedule can use up cartridges at different rates.

Do you need to change it more often in hard water?

Hard water does not change what a shower filter cartridge is designed to do. The DWI's guide to water hardness explains that hardness is dissolved calcium and magnesium, while a filter shower head is designed to reduce free chlorine. It is not a water softener, and it does not remove limescale.

Our suggestion depends on household size only, and we have no data linking hardness to how quickly a cartridge is used up. If limescale is your real concern, a water softener, not a shower filter, is the tool for hardness.

How do you know when to change a shower filter?

The reliable way to know when to change a shower filter is the date, because you usually can't tell by looking or by smell when a cartridge is spent. Note the day you fit a cartridge and plan the next change from there.

In our kit, the cartridge sits inside the handle, out of sight. Smell is a poor guide as well: as Bristol Water explains, the chlorine level at your tap already rises and falls with the season and the time of day. A change in smell is not a reliable sign either way.

A routine is easier to keep than a judgement call:

• Write the date on a calendar or in your phone the day you fit a cartridge.
• Set a reminder for the next change: 3 months ahead, or 2 for households of two or more.
• Keep the next refill sealed in its sachet until the day you fit it.
• On a refill plan, the email we send before every charge also works as a reminder.

If your household grows, move the reminder to every 2 months.

Waterhush's suggested schedule and refill options

Waterhush's suggested schedule is a new cartridge every 3 months for one person, or every 2 months for households of two or more. Both are suggestions, not promises of how long a cartridge lasts.

The refill plan is optional and follows the same rhythm. Every plan option runs until you cancel, with no minimum term; you can skip or cancel online before a refill ships, and we email before every charge. Prices include VAT and free tracked UK delivery.

The box of two is a delivery rhythm, not a 6-month cartridge: it holds two cartridges, one for each 3-month change. On the plan, the first refill ships 3 months after your kit (2 months on the 2-month rhythm), and the first box of two also ships at 3 months.

Each Waterhush refill cartridge is the same cartridge that comes fitted in the kit, sealed in its own airtight sachet. How the refill plan works sets out every option and term.

How to change a shower filter cartridge

Changing a shower filter cartridge means swapping the used cartridge for a new one, following the maker's leaflet. With our kit, the instruction leaflet in the box shows how to change the cartridge in the handle.

1. Check the date you noted when you fitted the last cartridge.
2. Turn the shower off.
3. Open the new refill's airtight sachet only when you are ready to fit it.
4. Follow the leaflet to take out the used cartridge and fit the new one.
5. Note the date and set a reminder for the next change.

Keep refills sealed until you use them. Under our returns policy for refills, unused refills can be returned as normal, and a refill that has been fitted or used may get a reduced refund because it can't be resold.

What happens if you don't change the cartridge?

If you don't change a shower filter cartridge, its media are eventually used up and it stops doing its job. The DWI's advice for home water filters is that a cartridge not changed in line with the maker's instructions will cease to be effective.

For a shower filter, that job is reducing free chlorine, and you probably won't notice the moment the media run out. That is why a date works better than a hunch.

If you have gone past the date, fit a new cartridge when you next can and count the next change from that day. Keeping to a schedule is a running cost, so it helps to know the yearly figure first: what a shower filter costs per year, worked out line by line.

What we don't know yet

What we don't know yet is the capacity of our cartridge: we have no figure for it, in litres or in showers, which is why our schedule is only a suggestion. Three other questions stay open:

• How many showers a cartridge covers in your home. That depends on your water use and your supply's chlorine level.
• Whether hardness changes how quickly a cartridge is used up. We have no data either way.
• The exact moment a cartridge is spent. You can't see the media, and smell is not a reliable sign.

Without those answers, the honest advice is a schedule you can keep.

The bottom line

Change a shower filter cartridge on its maker's schedule, because the media are used up as water runs through them. Waterhush suggests every 3 months, or every 2 months for households of two or more: a suggestion, not a promise of how long a cartridge lasts. Your water use and your supply's chlorine level make the real difference, and smell is no guide. Note the date you fit each cartridge and plan the next change from there.